1. Field of the Invention
The invention relates to a magnetic resonance apparatus.
2. Description of the Prior Art
Magnetic resonance technology is a known technique for obtaining images of the body interior of an object to be examined. Rapidly switched gradient fields that are produced by a gradient system are superimposed in a magnetic resonance apparatus on a static basic magnetic field that is produced by a basic field magnet system. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the object to be examined in order to trigger magnetic resonance signals, and picks up the triggered magnetic resonance signals, on the basis of which magnetic resonance images are generated.
The magnetic resonance signals are electromagnetic signals in the radio-frequency range, their exact wavelength range being a function of the strength of the basic magnetic field. For the reception, and thus for the magnetic resonance images not to be influenced by external radio-frequency interference as far as possible, it is customary in magnetic resonance technology for at least the actual basic field magnet of the basic field magnet system, a gradient coil system of the gradient system and antennas of the radio-frequency system to be arranged in a shielding cabin. In this case, the shielding cabin forms an uninterrupted enclosure of the above components of the magnetic resonance apparatus, having an attenuation, for example, of at least 90 dB for a frequency band from 10 to 200 MHz, and is, apart from windows, constructed from sheet steel or a copper foil, for example. Of course, the shielding cabin also develops its shielding effect in the reverse direction from inside outward. One embodiment of a shielding cabin is described in U.S. Pat. No. 4,651,099, the shielding cabin being constructed from ferromagnetic metal for the purpose of additional shielding, for example shielding a stray field of the basic field magnet from the outside.
Because a gradient amplifier for supplying the gradient coil system, and a control system, downstream of the antennas, of the radio-frequency system, are arranged outside the shielding cabin, connecting lines, for example between the gradient coil system and the gradient amplifier, are fed through via filters integrated in the shielding cabin, so that no radio-frequency interference can reach the shielding cabin. An embodiment such a filter is described in U.S. Pat. No. 6,218,836.
Appropriate currents need to be set in gradient coils of the gradient coil system in order to produce gradient fields. Amplitudes of the required currents are up to several 100 A. The current rise and fall rates are up to several 100 kA/s. Given the presence of a basic magnetic field on the order of magnitude of 1 T, these temporally varying currents in the gradient coils are acted on by Lorentz forces that lead to mechanical vibrations of the gradient coil system. These vibrations are passed on to the surface of the magnetic resonance apparatus via various propagation paths. There, the mechanical vibrations are converted into acoustic vibrations that finally lead to noise, which is undesired per se. Peak values of more than 125 dB are reached.
When an examination begins, a patient capable of normal reactions is given a pushbutton which, if activated during the examination, signals the occurrence of a problem to an operator working at a display and operating device of the magnetic resonance apparatus arranged outside the shielding cabin. Because it is advantageous to have the option of closer communication between the patient and the operator, is it known, for example, from German PS 195 24 847 to transmit to the operator acoustic signals coming from the patient. For this purpose German PS 195 24 847 discloses a device in which the effect of interference on the transmission quality is reduced. In this case, the device is constructed with at least two microphones, signals picked up by the microphones having a speech signal component and a disturbing noise component. The processing of the microphone signals is undertaken in this case with the aim of reducing the disturbing noise component in three frequency sub-bands. In a middle frequency band, the signal is weighted with a scalar factor such that this frequency band is damped during the speech pauses; the scalar weighting in the middle frequency band is set up on the basis of an estimated signal-to-noise ratio. In an upper frequency band, use is made of an adaptive filter that is calculated in a device by averaging from two filters adapted in terms of linear phase, the coefficients being spectrally smoothed. At the start of the processing an increase of the signal levels is performed that is cancelled again by an inverse filter before the output of the improved signal.
An object of the present invention is to provide an improved magnetic resonance apparatus with which acoustic signals coming from a patient being examined can be picked up in such a way that these signals can be freed from noise of the magnetic resonance apparatus also picked up.
The object is achieved according to the invention in a magnetic resonance apparatus having a first microphone that is arranged to pick up acoustic signals coming from a patient supported for an examination in the magnetic resonance apparatus, at least one second microphone that is arranged so that it is as free as possible from picking up the acoustic signals, and a processing unit to which the signals of the microphones are fed, which eliminates noise of the magnetic resonance apparatus also picked up, for the purpose of extracting the acoustic signals.
The described arrangement of the two microphones ensures that the acoustic signals from the patient, despite the very loud noise of the magnetic resonance apparatus superimposed on the acoustic signals, can be picked up, transmitted and, for example, made available to an operator of the magnetic resonance apparatus in a way that is clear and distinct and freed of noise.
In an embodiment, the processing unit includes a subtractor with the aid of which the two microphone signals can be subtracted from one another such that the noise of the magnetic resonance apparatus also picked up is eliminated for the purpose of extracting the acoustic signals from the patient.
Owing to the different attachment locations of the microphones, the noise picked up by the two microphones, from the magnetic resonance apparatus can differ in the microphone signals. In an embodiment the signal from the second microphone is filtered upstream of the subtraction with the aim of fitting it as closely as possible to the noise picked up by the first microphone.
Since the difference between the noise included in the two microphone signals is not determined exclusively by the attachment sites of the two microphones, but is also a function of the particular surroundings of where the magnetic resonance apparatus is set up, and of the acoustics associated therewith, the filtering can advantageously be carried out with an adaptive filter.